Field of the Invention
The present invention relates generally to vapor deposition processes and, more particularly, to deposition of metal carbide films by vapor deposition processes.
Description of the Related Art
Metal carbides have found use in various applications in the electronics industry, from gate electrodes to diffusion barriers. For example, tantalum carbide (TaC) is a low resistivity metal that can be used as an n-type metal oxide semiconductor (NMOS) gate electrode. Further, TaC has been found to be effective at inhibiting electromigration of noble metal atoms at the interface between metal interconnects and metal lines.
Generally, carbides of transition metal elements are in groups 4, 5, 6, 7, 8, 9, and 11 of the periodic table. Transition metal carbides are relatively inert, have very high melting points, are extremely hard and wear resistant, and have high thermal conductivity and metal-like electrical conductivity.
Transition metal carbides can have a wide range of compositions. Ordered and disordered carbon deficient forms exist, of which the tungsten carbides, WCx, are examples. In these forms, carbon resides in the interstitial cavities between metal atoms.
Metal carbide films have been formed by various methods including chemical vapor deposition (CVD), physical vapor deposition (PVD) and atomic layer deposition (ALD).
A “thermal” ALD method of forming metal carbide films, wherein the substrate is sequentially and alternately contacted with vapor phase pulses of two or more source chemicals, is described in, for example, U.S. Pat. No. 6,482,262. According to the methods described therein, a transition metal source chemical and carbon source gas are alternately and sequentially pulsed into a reaction space comprising a substrate, which is maintained at an elevated temperature. The pulsing sequence is repeated to form a metal carbide (e.g., TaC) film of desired thickness. Due to the self-limiting nature of ALD, films are grown at rate of about one monolayer (ML) per deposition cycle.
A CVD method of depositing tungsten carbide from tungsten hexafluoride, hydrogen and a carbon-containing gas has been described in, for example, international patent application WO 00/47796. The carbon-containing compound is initially thermally activated. All of the gaseous source chemicals are introduced into a reaction space at the same time, resulting in the deposition of nonvolatile tungsten carbide on the substrate. A CVD reaction of WF6 with trimethylamine and H2 has been disclosed to yield WC films at 700° C.-800° C. and beta-WCx films at 400° C.-600° C. (Nakajima et al., J. Electrochem. Soc. 144 (1997) 2096-2100). The H2 flow rate was found to affect the deposition rate of the tungsten carbide films. A problem with the disclosed process is that the substrate temperature is rather high relative to thermal budgets for state-of-the-art semiconductor fabrication, particularly in metallization stages.
PVD processes generally deposit along a line-of-sight. One method of depositing tantalum carbide for a diffusion barrier layer by PVD has been described in U.S. Pat. No. 5,973,400. A tantalum carbide layer was formed by sputtering tantalum or tantalum carbide under an N2/CH4/Ar atmosphere. Line-of-sight deposition, however, means that complex substrate contours will have insufficient film coverage in shaded areas. Additionally, line-of-sight deposition means that low-volatility source material arriving directly from the source to the substrate will likely adhere to the first solid surface that it encounters, thus producing low-conformality coverage.